I don't think Washington said that. Someone mentioned the Treaty of Tripoli earlier in this thread, and checking a little on the web I found that that quote, or something close to it, does indeed come from the Treaty of Tripoli, signed in 1796 while Washington was still president.
Speaking now to everyone here, like any large group of people I expect the founder's religious views were varied and probably highly nuanced, but there can be no doubt that they were wary of state sponsored religion. Modern interpretations of the establishment clause may have gone a bit too far (e.g., high school Bible study clubs have been challenged on 1st amendment grounds), but the 1st amendment is sufficiently unambiguous that even legal rulings by the Supreme Court could not significantly weaken it. Meaningful inroads toward making the United States an officially Christian nation would require its repeal.
So Christians can doctor up paintings that include our founders with Christian images, and secularists can misattribute quotes, and both will be successful at convincing people one way or the other, but only if Christians are so successful that the 1st amendment is repealed will the United States ever become a Christian nation.
But I'm still interested in the facts of the matter. I think both sides can agree that Thomas Jefferson would definitely not side with those claiming the United States is a Christian nation, but what about other founders such as Washington, John Adams, Alexander Hamilton, James Madison and Benjamin Franklin, among others.
--Percy